WDM optical transmission systems employ a variety of different passive components. Such components are increasingly being fabricated on Planar Light-Guide Circuits (PLC). A planar lightguide circuit, also known as an optical integrated circuit, can be readily mass produced because the processing steps are compatible with those used in silicon integrated circuit (IC) technology, which are well known and geared for mass production.
One common type of planar lightguide circuit employs doped-silica waveguides fabricated with silicon optical bench technology. Doped-silica waveguides are usually preferred because they have a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber. Such a planar lightguide circuit is fabricated on a carrier substrate, which typically comprises silicon or silica. The substrate serves as a mechanical support for the otherwise fragile lightguide circuit and it can, if desired, also play the role of the bottom portion of the cladding. In addition, it can serve as a fixture to which input and output fibers are attached so as to optically couple cores of an input/output fiber to the cores of the planar lightguide circuit. The fabrication process begins by depositing a base or lower cladding layer of low index silica on the carrier substrate (assuming the substrate itself is not used as the cladding layer). A layer of doped silica with a high refractive index, i.e., the core layer, is then deposited on top of the lower cladding layer. The core layer is subsequently patterned or sculpted into structures required by the optical circuits using photo-lithographic techniques similar to those used in integrated circuit fabrication. Lastly, a top cladding layer is deposited to cover the patterned waveguide core. This technology is well known and is generally described, for example, in U.S. Pat. No. 4,902,086 issued to C. H. Henry et al., and in an article entitled “Glass Waveguides on Silicon for Hybrid Optical Packaging” at pp. 1530–1539 of the Journal of Lightwave Technology, Vol. 7, No. 10, October 1989.
One important passive component that can be fabricated on a PLC is an optical “star coupler” in which waveguides are radially positioned on opposite sides of a slab waveguide. As used in the present invention, a slab waveguide means a planar area, which is large compared to the area of an individual waveguide of the same length, that supports lightwave transmission between input and output waveguides. Optical power entering the slab from input waveguides on one side of the slab is conveyed to output waveguides on the other side. (Ideally, the power is distributed equally among all of the output waveguides.) In an M×N star coupler, for example, the optical power carried by each input waveguide is transmitted across the slab and distributed among the N output waveguides, which are generally arranged in an array. However, if the waveguides in the output array are not well coupled (which is generally the case for star couplers in a so-called “Dragone” router because of the gaps between the array waveguides), then there is a loss of power due to the scattering of light at the junction between the array and the slab. Such losses comprise a major portion of the router's insertion loss.
One technique for reducing the insertion loss of an optical device, such as the aforementioned Dragone router, is taught in a paper entitled “Loss Reduction for Phased-Array Demultiplexers Using a Double Etch Technique,” which was published in Integrated Photonics Research, Technical Digest Series, Vol. 6, Apr. 29–May 2, 1996. In this technique, a transition region having a shallow etch depth is inserted at the junction between the slab and the array waveguides. As might be expected, coupling between adjacent waveguides is improved and coupling losses are decreased. Nevertheless, a greater reduction in insertion loss is desired, and the double etch technique adds a processing step.
Another technique for reducing insertion loss in an optical device is disclosed in U.S. Pat. No. 5,745,618. In this reference insertion loss is reduced between the slab waveguide and the output waveguide array of a star coupler by providing the output waveguide array with a transition region. The transition region includes a number of silica paths that intersect the output waveguide array. One problem with this approach is that it is difficult to manufacture the resulting device because the many small areas adjacent to the intersections between the output waveguides and the silica paths must be completely filled with cladding material, which is difficult to accomplish because of their small dimensions.
Accordingly, it would be desirable to provide an improved technique for reducing insertion loss in an optical device that does not involve additional processing steps and which can be implemented in a highly reliable manner.